1.4 Sea-Floor Spreading

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Vocabulary

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Mid-ocean ridge: the longest chain of mountains in the world
Sonar: a device that sends sound waves that bounces off an object then records the echoes
Sea-Floor spreading: the process which continously adds molten material to the ocean floor
Deep ocean trenches: deep under water canyons
Subduction: the process in which the sea floor sinks into the mantle

Outline

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Mapping the Mid-ocean ridge

• Scientists used a sonar to map the Mid-ocean ridge
  • Sonars are usually already assembled in ships
• the ridge curves extending into a oceans
  • there are some places where part of the ridge sticks up above the surface

Evidence for Sea-Floor Spreading

• Evidence from molten material
  • a couple of scientists took a dive in a submarine and found out that the presence of some rocks showed that molten material has erupted again and again making this mountain longer.
• Evidence from magnetic strips
  • when scientists studied the rocks of the ocean floor they found a pattern
    • this pattern of stripes hold a record of reversals in Earth's magnetic field
• Evidence from drilling samples
  • a ship took some drilling pipes and got some drilling samples of the Mid-ocean ridge
    • the drilling samples proved the molten material keeps on erupting( the rocks that were older were found farther away from the ridge than the newer ones that were closer to the ridge.

Subduction at Deep Ocean Trenches

• the ocean floor can't get wider and wider
  • parts of the floor gets sucked back into the mantle

Subduction and Earth's Oceans

• Subduction in the Pacific Ocean
  • if a deep trench swallows too much oceanic crust and the Mid-ocean ridge can't produce enough oceanic crust, the width of the ocean will shrink
• Subduction in the Atlantic Ocean
  • on the other hand the A. Ocean is expanding
    • since the A. Ocean doesn't have that many deep ocean trenches it starts to expand

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